If one spouse has already maxed out the credit while filing single, it can't be taken again when filing jointly. The same goes for initially claiming the credit while filing jointly, and later wanting to claim the credit filing single.
See the Lifetime Limitation Worksheet—Line 18 in the Instructions for Form 5695. It makes no distinction as to a taxpayer's current or previous filing status, and uses the limitation amount from the worksheet on the form, itself.
